This invention relates to a method of producing a powder of high purity zirconia, which may be unstabilized zirconia or stabilized or partially stabilized zirconia, by heating a zircon powder in the presence of carbon.
Zirconia, i.e. zirconium oxide ZrO.sub.2, has been used as a refractory material in the steel manufacturing and glass manufacturing industries since the melting point of this material exceeds 2700.degree. C. Recently, zirconia is acquiring new and widening uses in various fields. For example, it serves as an important raw material for producing optical glasses, new abrasives, piezoelectric elements, solid electrolyte cells useful as oxygen sensors, ceramic capacitors or some other electronic devices.
Crystallographically, pure zirconia belongs to the monoclinic system at normal temperature. However, it is not fully stable and reversibly transforms into a tetragonal phase crystal structure at about 1100.degree. C. with a relatively large change in volume. Accordingly, practically pure zirconia is commonly called unstabilized zirconia. Due to the change in volume accompanying the transformation, a sintered body of unstabilized zirconia is not high in mechanical strength even at normal temperature. As a solution to this problem, it is well known to obtain stabilized or partially stabilized zirconia having a cubic phase crystal structure by the addition of a stabilizing oxide such as, e.g., CaO or Y.sub.2 O.sub.3 which forms a solid solution with ZrO.sub.2. Besides the aforementioned uses of zirconia, stabilized or partially stabilized zirconia which is high in both strength and toughness has prospective uses in the field of so-called engineering ceramics.
A popular raw material for producing zirconia is zircon, which is a zirconium silicate mineral represented by the formula ZrO.sub.2.SiO.sub.2 or ZrSiO.sub.4 and decomposes into ZrO.sub.2 and SiO.sub.2 at about 1530.degree. C. Therefore, producing zirconia from zircon means separating silica from zircon.
A well known method for producing zirconia from zircon is an arc furnace fusion method in which a mixture of zircon sand, carbon and iron scrap, with the addition of a stabilizing oxide when producing stabilized zirconia, is heated in an arc furnace. Decomposition and reduction reactions and fusion of the reaction system take place, and the Si.sub.2 component separates from the ZrO.sub.2 component by dissipation of the silicon oxide in reduced form into the gas phase and/or by formation of ferrosilicon by reaction with iron. This method is capable of producing zirconia at a relatively low cost and is suited to the production on a large scale. However, this method cannot provide high purity zirconia, and zirconia as the product of this method is in the form of a hard block which needs to be pulverized with consumption of large energy.
Another known method is an alkali fusion method. In this method, a mixture of zircon sand and an alkali is melted to undergo reaction which gives an alkali silicate and an alkali zirconate. The silicates can be washed away. By a process including an acid treatment the zirconate is converted into zirconium oxychloride ZrOCl.sub.2, which is soluble in water and therefore can easily be converted into zirconium hydroxide. Heat treatment of thus obtained zirconium hydroxide gives zirconia in powder form. By this method it is possible to produce very high purity zirconia, but this method is low in productivity and very high in the production cost because of employing a roundabout process including many kinds of reactions.
As a still different method, Japanese patent applications publication Nos. 58-9808 and 58-15021 propose to accomplish desiliconizing of zircon by heating a granulated mixture of zircon sand and carbon powder, with the addition of a stabilizing oxide if desired, in a nonoxidizing atmosphere in the presence of granular carbon which is disposed adjacent to the granules of the raw material mixture. By this heat treatment the silica component of zircon gasifies in a reduced form and then reacts with the granular carbon. Accordingly silicon carbide is obtained together with zirconia. However, this method is relatively low in productivity and high in the energy cost because the reactions in this method need to be carried out at a very high temperature and for a long period of time, and it is difficult to obtain zirconia of high purity.
Also U.S. Pat. No. 4,118,464 shows a similar method for producing zirconia together with silicon carbide. A powder mixture of zircon and carbon is pelletized and surrounded with a matrix of another solid phase of additional carbon. In that state a heat treatment is made in a nonoxidizing atmosphere to reduce silica contained in zircon to silicon monoxide in gaseous phase and to allow the gaseous silicon monoxide to diffuse into and react with the surrounding carbon phase to form silicon carbide. According to the description of an example of this method the heat treatment was performed at about 1485.degree. C. for 250 min, resulting in that the reaction was 98.8% complete and produced zirconia with a slight amount of zircon. This is illustrative of the difficulty of accomplishing complete desiliconization of zircon.